Calculate the number of atoms in each of the following

Solution:

(i) 1 mole of $A r=6.022 \times 10^{23}$ atoms of $A r$

$\therefore 52 \mathrm{~mol}$ of $\mathrm{Ar}=52 \times 6.022 \times 10^{23}$ atoms of $\mathrm{Ar}$

$=3.131 \times 10^{25}$ atoms of $A r$

(ii) 1 atom of $\mathrm{He}=4 \mathrm{u}$ of $\mathrm{He}$

Or,

4 u of He = 1 atom of He

$1 \mathrm{u}$ of $\mathrm{He}=\frac{1}{4}$ atom of $\mathrm{He}$

$52 \mathrm{u}$ of $\mathrm{He}=\frac{52}{4}$ atom of $\mathrm{He}$

$=13$ atoms of He

(iii) $4 \mathrm{~g}$ of $\mathrm{He}=6.022 \times 10^{23}$ atoms of $\mathrm{He}$

$\therefore 52 \mathrm{~g}$ of $\mathrm{He}=\frac{6.022 \times 10^{23} \times 52}{4}$ atoms of $\mathrm{He}$

$=7.8286 \times 10^{24}$ atoms of $\mathrm{He}$